The present invention is directed to an micromachined infrared absorption emitter/sensor for detecting the presence of specific chemical and/or biological species.
This invention relates in general to micromachined infrared emitter sensors or a xe2x80x9csensor-on-a-chipxe2x80x9d and in particular to micromachined infrared emitter/bolometer sensors for detecting and discriminating the presence of specific biological, chemical, etc. substances comprising a heated bolometer integrated circuit element as a source of infrared emission, a filter for controlling the wavelength of emitted light and a detector of the absorption of the emitted light by a substance interacting with the emitted light.
There is a very serious demand and need for low-cost, mass market gas and chemical sensors, such as, for example, indoor natural gas, radon and carbon monoxide (CO) sensors. In the United States alone nearly 300 people die and thousands are injured from unintentional carbon monoxide poisoning every year. Such mass market sensors must be both hardy and sensitive. For example, CO concentrations of only 50 ppm can produce symptoms of carbon monoxide poisoning over a period of time while CO concentrations of 2000 to 2500 ppm will produce unconsciousness in about 30 minutes and higher CO concentrations can kill. As a comparison, typical gasoline-powered auto exhaust contains anywhere between 300 to 500 ppm concentrations of CO. The need for natural gas sensors, meanwhile, was highlighted most recently in the devastating fires that followed earthquakes in Northridge, Calif. and Kobe, Japan leading to a call for natural gas distribution systems to incorporate sensors in combination with automatic shut-off valves.
Currently the market for small, low-cost CO sensors is served by either catalytic or electrochemical sensors. Catalytic sensors use optical measurements to observe chemical, enzyme or bioengineered coatings that react, very specifically, to a substance of interest such as, for example, carbon monoxide. Despite the sensitivity and specificity of these detectors, inherent limitations reduce their utility in a mass market. For example, the catalytic element on these sensors requires periodic replacement, raising use cost and increasing the likelihood that the sensor will fail as a result of poor maintenance or high levels of contaminants.
Electrochemical sensors measure a change in output voltage of the sensing element due to interaction of the species of interest on the sensing element. While these electrochemical sensors are inexpensive and very sensitive, they are also historically subject to interference and false alarms due to chemical species other than that sought interacting with the sensing element. In addition, these sensors respond slowly and the response is not always reversible. Indeed, exposures to high concentrations of the species of interest can result in a permanent shift of the zero-point requiring a re-calibration of the unit. Furthermore, temperature and humidity changes frequently cause drift and false readings, and outgassing from the plastic and cardboard in which the detectors are packaged can also contaminate the sensors prior to actual sale to the consumer. Moreover, in many of these devices the detector element must be heated, and current consumer models require about 5 watts of continuous power. Although the cost of such usage per annum is low, these sensors"" reliance on a steady source of power results in sensor failure if there is a power outage, when the sensor may be needed the most.
Despite these limitations, over 20 million American homes have installed CO monitors utilizing either a catalytic or electrochemical sensor. However, recently, a number of articles have appeared pointing out that a very high percentage of alarms triggered by available CO sensors are false alarms and that a very high percentage of sensors don""t set off alarms when appropriate. See, e.g., xe2x80x9cHome Alarms for Carbon Monoxide Recalledxe2x80x9d, Washington Post, Mar. 19, 1999; xe2x80x9cULC Investigation Indicates Failures of Certain Lifesaver and Nighthawk CO Detectorsxe2x80x9d, Canada Newswire, www.newswire.ca/releases/Mar. 19, 1999/ c5815.html; xe2x80x9cAmeriGas fined, must give free carbon monoxide detectors,xe2x80x9d Manchester (N.H.) Union Leader, Apr. 9, 1999; xe2x80x9cFalse Alarmsxe2x80x9d, Forbes Magazine, Jan. 13, 1997; xe2x80x9cCarbon Monoxide Alarms Recalledxe2x80x9d, USA Today, Mar. 19, 1999. The Gas Research Institute estimates that more than 80% of emergency calls triggered by CO sensors are false alarms and as many as 20% of the CO sensors sold in 1999 were recalled as defective.
One avenue of sensor development currently being investigated uses diode lasers for optical detection techniques. While this technique is again highly sensitive and less subject to contamination and false alarms than catalytic or electrochemical sensors, the units presently cost too much for home installation. In addition, because they depend on physical band-gaps, diode lasers can only be tuned with difficulty over a very narrow range. Moreover, there are no uncooled diode lasers and only low efficiency, low output (xcx9c5 xcexcW), expensive (xcx9c$450) LEDs available at the wavelengths (xcx9c2-6 xcexcm) for gas sensing.
Another detector technology currently under study utilizes infrared spectroscopy to detect species of interest. Many hazardous and pollutant gases (e.g., volatile organic compounds, carbon dioxide, nitrogen oxides, and sulfur dioxide) have unique infrared absorption signatures in the 2 to 12 xcexcm region of the infrared. In general, infrared absorption is a function of the wavelength, gas concentration, temperature and pressure such that if the concentration of the species of interest is low enough to be considered dilute, then the absorption is directly proportional to the concentration. In addition, by observing a reference wavelength corrections can be easily made for contaminants, such as, for example, dust. While sensors designed to take advantage of the sensitivity and resolution of infrared spectroscopy are well-known in the art and are frequently used for industrial application, such as, for example, automotive exhaust, refrigerants and glucose monitors, the size and complexity of the infrared sensor unit has precluded their use in the mass-market. Conventional infrared gas and chemical sensors are expensive, high performance units consisting of a cabinet full of discrete components. For example, one type of conventional infrared sensor employs a multi-component design. In this design an infrared light source, usually a blackbody emitter, such as, for example, a Nernst glow bar or tungsten filament modulated by a mechanical chopper, serves as a source of infrared radiation. The radiation is directed through a sample compartment containing the sample gas or liquid to be measured or tested and then the radiation is directed to a separated monochromator and infrared detector and amplifier. The radiation is analyzed as intensity vs. wavelength, either by a spectrometer or by detectors with narrow-band interference filters. Much of the bulk and cost of these conventional infrared instruments is designed to maintain optical alignment in the face of varying ambient conditions and in spite of the expense and effort these instruments frequently require re-calibration and/or realignment.
Recently, photonic band gap structures, such as periodic dielectric arrays, have received much attention as optical and infrared filters with controllable narrow-band infrared absorbance. These photonic structures have been developed as transmission/reflection filters, low-loss light-bending waveguides, and for inhibiting spontaneous emission of light in semiconductors which could lead to zero-threshold diode lasers. In principle these photonic band gap structures operate as follows: electromagnetic waves with wavelength on the order of the period of the dielectric array propagate through this structure, the light interacts in a manner analogous to that for electrons in a periodic symmetric array of atoms. Thus the structure exhibits allowed and forbidden extended states, a reciprocal lattice, Brillouin zones, Bloch wavefunctions, etc. Recently these structures have even been used to create narrow-band infrared radiators. Emissivity of these metal filaments is controlled by creating random surface texture (sub-micron scale rods and cones) which modifies the surface absorption spectrum. Incoming light of wavelengths that are small compared to the feature sizes are scattered from the surface, producing high emissivity, whereas light of wavelengths that are long compared to the feature sizes are not scattered, producing low emissivity. Accordingly, by controlling the average feature size at the surface of these photonic bandgap structures, the wavelength of the emitted light can be controlled. One such infrared radiator is manufactured and distributed by IonOptics under the name pulsIR(copyright). The pulsIR(copyright) infrared emitter utilizes an ion beam etched-randomly textured surface structure which shows increased optical adsorption over a defined wavelength range and also shows preferential emission over the same waveband when heated. The emitted infrared spectrum is essentially a modified black-body spectrum and provides far more infrared signal for a fixed power input within a narrow infrared band than standard black-body lamps.
Despite the promise that photonic band gap structures exhibit in terms of size and stability, most researchers have only utilized them as tuned absorbers or filters and not as emitters. Accordingly, significant limitations exist in the quantum efficiency and output power of currently available photonic band gap emitters which emit in a wavelength band typically of xcex94xcex/xcexxcx9c0.5, limiting their feasibility for use in portable battery operated systems. Additionally, the photonic bandgap emitters currently produced require a separate infrared emitter, detector, wavelength filter and optics to make spectroscopic measurements, driving up the cost and complexity of the resulting infrared sensor system.
Accordingly, a need exists both for a low-cost mass market sensor system capable of accurately and sensitively detecting and discriminating the presence of specified substances in the environment and for an improved infrared sensor capable of meeting the demands of the mass market.
The present invention is directed to a device and system for utilizing an optical infrared emitter/detector to sensitively sense substances of interest. This invention utilizes a photonic bandgap structure which functions both as an infrared emitter, a narrow-band filter, and as a broad-band infrared bolometer detector to sense the presence of a specified substance in the environment. This invention also uses the photonic band gap structure to exert wavelength control directly on the active element emitter/detector surface using the periodic symmetry of the photonic band gap structure to produce narrow wavelength xe2x80x9cforbiddenxe2x80x9d optical transmission bands or modes. This invention is also directed to systems for integrating the optical infrared emitter/detector of this invention into a device for sensing specific substances. This invention is also directed to novel methods for detecting a wide range of substances using the infrared emitter/detector of the invention. This invention is also directed to a method of manufacturing the infrared emitter/detectors of the present invention.
In one embodiment, the optical infrared emitter/detector of the present invention is incorporated into an infrared sensor comprising a thermally isolated version of the narrow-band emitter/bolometer detector of the current invention and a reflector. The narrow-band emitter/bolometer detector is designed to emit a narrow-band of infrared light and detect a change in the temperature of the infrared light reflected back onto the emitter/detector by the reflector. The emitter/detector is placed in line-of-sight with the reflector such that the intervening space between the emitter/detector and reflector comprises the optical cell. The emitter/detector then projects a beam of infrared light across the optical cell to the reflector, the reflector then sends the light back toward the emitter/detector. In the absence of any absorption in the optical cell by the species of interest, the filament and optics quickly reach a thermal equilibrium. Absorbing gas in the optical cell will reduce the reflected optical power returning to the element and it will reach equilibrium with its surroundings at a slightly lower temperature. This change in the equilibrium temperature is detected as a change in the resistance of the emitter/detector bolometer. The emitter/detector is modified such that it emits infrared light in a narrow wavelength band in the spectral region in which the substance of interest absorbs.
In such a sensor system, the emitter/detector comprises a substrate having a thin, non-random, periodic array of etched metal (or photonic band gap (PBG) structure) atop the emitting surface of the substrate wafer such that the wavelength of emitted light from the emitter surface is proportional to the spacing of the geometric patterns of the non-random periodic array of etched metal on the substrate""s surface. The substrate wafer is preferably made of a material having a high temperature coefficient of resistance such that a small change in equilibrium temperature results in a disproportionately large shift in substrate resistance.
In a preferred embodiment, the substrate is made of a semiconductor such as, for example, silicon and the periodic array is preferably made of a conducting metal such as gold. In this embodiment the silicon can be further doped to adjust the final device resistance and therefore the required drive current and battery life.
In another preferred embodiment, the substrate is made of single crystal semiconductor having a resistance that has an exponential dependence on the temperature of the filament such as, for example, silicon. Utilizing such a material, allows for far more sensitive bolometric detection than a similar detector based on metals, which generally exhibit a linear dependence of resistance with temperature.
In another preferred embodiment, the size, shape and pattern of the photonic band gap structure etched on the substrate wafer is adapted such that the absorbance of the sensor is enhanced in a narrow-band wavelength corresponding to the absorption wavelength for a species of interest, such as, for example, CO at a wavelength between 4.65 and 3.9 xcexcm.
In yet another preferred embodiment, the infrared sensor of the present invention further comprises a reflective optic adapted such that infrared light emitted from the emitter/detector is collimated into a concentrated beam of light prior to entering the optical cell and infrared light reflected back into the emitter/detector from the reflector is refocused prior to reaching the emitter/detector. In this embodiment a preferred reflective optic is a compound parabolic concentrator.
In yet another preferred embodiment the infrared sensor system of the present invention comprises an emitter/detector in signal communication with an monitoring device such that when the emitter/detector detects the presence of the species of interest a signal is sent to the monitoring device, such as, for example, a programmable chip in signal communication with an audible alarm.
In still yet another embodiment, the invention is directed to a system for the detection of substances comprising multiple emitter/detectors as described above, adapted to either detect the same or different species of interest each of which is in signal communication with at least one monitoring device as described above.
In still yet another additional embodiment, the invention is directed to a method for detecting and discriminating a substance in contact with the infrared sensor. The method comprises analyzing the air in an environment using a infrared sensor as described above.
In still yet another additional embodiment, the invention is directed to a method for manufacturing the infrared sensor as described above. The method comprising manufacturing the emitter/sensor using conventional microelectromechanical (MEMS) manufacturing techniques, such as, for example, electron beam lithography techniques.
In still yet another embodiment, the invention is directed to a method for manufacturing the infrared sensor as described above. The method involves the use of silicon-on-insulator (SOI) or silicon-oxide-silicon (SOS) substrates, whereby single crystal silicon films are used to produce the high sensitivity detectors.